The present invention relates to a solid polymer electrolyte comprising a sulfonic acid group-containing polymer excelling in oxidation resistance, hot water resistance, and the like which is used for fuel cells, water electrolysis, salt electrolysis, humidity sensors, gas sensors, and the like, and to a proton-conducting membrane comprising the electrolyte.
A solid polymer electrolyte is a solid polymer material including an electrolyte group such as a sulfonic acid group or a carboxylic acid group in the polymer chain. Since the solid polymer electrolyte has properties of strongly bonding to a specific ion or selectively allowing a cation or anion to permeate therethrough, the solid polymer electrolyte is utilized as a polymer electrolyte membrane for polymer electrolyte fuel cells or water electrolysis cells.
In a polymer electrolyte fuel cell, a pair of electrodes is provided, with one on each side of a proton-conducting solid polymer electrolyte membrane. Pure hydrogen or reformed hydrogen gas is supplied to one electrode (fuel electrode) as fuel gas, and oxygen gas or air is supplied to the other electrode (air electrode) as an oxidizing agent to obtain electromotive force. In water electrolysis, hydrogen and oxygen are produced by electrolyzing water by using a solid polymer electrolyte membrane to cause the reverse reaction of the fuel cell reaction to occur.
However, a side reaction occurs in the fuel cell or water electrolysis in addition to the main reaction. A typical side reaction is generation of hydrogen peroxide (H2O2). A radical species originating in hydrogen peroxide causes the solid polymer electrolyte membrane to deteriorate.
As the solid polymer electrolyte membrane, a perfluorosulfonic acid membrane commercially available under the trade name of NAFION (registered trademark, manufactured by DuPont), ACIPLEX (registered trademark, manufactured by Asahi Kasei Corporation), or FLEMION (registered trademark, manufactured by Asahi Glass Co., Ltd.) has been used because of its chemical stability.
However, since the perfluorosulfonic acid electrolyte membrane is very expensive due to the difficulty in manufacture, its applications are limited to special uses. This hinders applications to commercial uses such as automotive and domestic fuel cells. Moreover, since the perfluorosulfonic acid electrolyte membrane contains a large number of fluorine atoms in the molecule, disposal after use poses serious environmental problems.
Therefore, as a less expensive solid polymer electrolyte membrane containing no fluorine atom, a polymer produced by sulfonating the main-chain aromatic ring of polyether ether ketone, polyether sulfone, or polyphenylene sulfide has been proposed. However, the polymer in which the main-chain aromatic ring is sulfonated not only has inferior hot water resistance due to high water absorption, but also has inferior Fenton resistance (radical resistance) which is considered to be the measure of power generation durability.